I am talking about a multi-column text frame. Let's say it has 3 columns, all equal widths. I'd like to change the column on the left to take up with width of the left and middle column, leaving the right column width as is.

And, unfortunately, it's also not possible in a multi-column frame to ahve columns of different widths. It you want to do that you need to use threaded frames. In this case you can draw one frame two columns wide, and thread it to a single column frame.